The Data Encryption Algorithm (DEA) is a complete description of a mathematical algorithm for encryption (encipher) and decryption (decipher) of binary coded data. Encryption converts plain text to unintelligible form and decryption converts ciphered text back to plain text. The DEA specifies both encryptions and decryption operations under a key. Plain text can be recovered for a given ciphered text, by using the same key that was used for encryption. As this algorithm is public knowledge, the security is achieved solely from the key protection.
The DEA standard has been developing for many years and International Business Machines Corporation has used a Cryptographic Algorithm defined by 1. Key.sub.-- Controlled Cryptographic Algorithm, Implementation of Cryptography, IBM Corporate standard specification, C-S 0-3031-001, 1989-03; and 2. Key.sub.-- Controlled Cryptographic Algorithm Description, IBM Corporate specification, C-H-0-3031-002, C-S 0-3031-001 since 1989 and 1990. These documents show how to implement and follow the standard which we have found needs improvement.
It will be noted that sometimes encryption and decryption are distinguished, but as the invention here works for both, the term encryption is used as the generic term.